diff --git a/src/Arr.php b/src/Arr.php
index f918334..2a06616 100644
--- a/src/Arr.php
+++ b/src/Arr.php
@@ -8,9 +8,9 @@ class Arr
      *Returns true if the provided function returns true for all elements of an array, false otherwise.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
      */
-    public static function all(array $items, callable $func): bool
+    public static function all(array $items, $func): bool
     {
         return \count(array_filter($items, $func)) === \count($items);
     }
@@ -19,9 +19,9 @@ public static function all(array $items, callable $func): bool
      * Returns true if the provided function returns true for at least one element of an array, false otherwise.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
      */
-    public static function any(array $items, callable $func): bool
+    public static function any(array $items, $func): bool
     {
         return \count(array_filter($items, $func)) > 0;
     }
@@ -71,9 +71,11 @@ public static function drop(array $items, int $n = 1): array
      * Returns the last element for which the provided function returns a truthy value.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
+     *
+     * @return mixed
      */
-    public static function findLast(array $items, callable $func)
+    public static function findLast(array $items, $func)
     {
         $filteredItems = array_filter($items, $func);
 
@@ -84,9 +86,9 @@ public static function findLast(array $items, callable $func)
      * Returns the index of the last element for which the provided function returns a truthy value.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
      */
-    public static function findLastIndex(array $items, callable $func)
+    public static function findLastIndex(array $items, $func)
     {
         $keys = array_keys(array_filter($items, $func));
 
@@ -116,7 +118,7 @@ public static function flatten(array $items): array
      * Groups the elements of an array based on the given function.
      *
      * @param array $items
-     * @param [type] $func
+     * @param mixed $func
      */
     public static function groupBy(array $items, $func): array
     {
@@ -139,12 +141,12 @@ public static function groupBy(array $items, $func): array
      * Sorts a collection of arrays or objects by key
      *
      * @param array $items
-     * @param [type] $attr
-     * @param [type] $order
+     * @param mixed $attr
+     * @param string $order
      *
      * @return array
      */
-    public static function orderBy(array $items, $attr, $order): array
+    public static function orderBy(array $items, $attr, string $order): array
     {
         $sortedItems = [];
         foreach ($items as $item) {
@@ -176,6 +178,8 @@ public static function hasDuplicates(array $items): bool
      * Returns the head of a list.
      *
      * @param array $items
+     *
+     * @return mixed
      */
     public static function head(array $items)
     {
@@ -186,6 +190,8 @@ public static function head(array $items)
      * Returns the last element in an array.
      *
      * @param array $items
+     *
+     * @return mixed
      */
     public static function last(array $items)
     {
@@ -196,7 +202,7 @@ public static function last(array $items)
      * Retrieves all of the values for a given key:
      *
      * @param array $items
-     * @param [type] $key
+     * @param mixed $key
      */
     public static function pluck(array $items, $key)
     {
@@ -209,7 +215,7 @@ public static function pluck(array $items, $key)
      * Mutates the original array to filter out the values specified.
      *
      * @param array $items
-     * @param [type] ...$params
+     * @param mixed ...$params
      */
     public static function pull(&$items, ...$params)
     {
@@ -222,9 +228,9 @@ public static function pull(&$items, ...$params)
      * Filters the collection using the given callback.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
      */
-    public static function reject(array $items, callable $func)
+    public static function reject(array $items, $func)
     {
         return array_values(array_diff($items, array_filter($items, $func)));
     }
@@ -233,9 +239,9 @@ public static function reject(array $items, callable $func)
      * Removes elements from an array for which the given function returns false.
      *
      * @param array $items
-     * @param callable $func
+     * @param mixed $func
      */
-    public static function remove(array $items, callable $func)
+    public static function remove(array $items, $func)
     {
         $filtered = array_filter($items, $func);
 
@@ -267,7 +273,7 @@ public static function take(array $items, int $n = 1): array
      * Filters out the elements of an array, that have one of the specified values.
      *
      * @param array $items
-     * @param [type] ...$params
+     * @param mixed ...$params
      */
     public static function without(array $items, ...$params)
     {
diff --git a/src/Str.php b/src/Str.php
index 32aa72b..3924cc3 100644
--- a/src/Str.php
+++ b/src/Str.php
@@ -82,7 +82,7 @@ public static function isAnagram(string $string1, string $string2): bool
     /**
      * Returns true if the given string is lower case, false otherwise.
      *
-     * @param [type] $string
+     * @param string $string
      *
      * @return bool
      */
@@ -94,11 +94,11 @@ public static function isLowerCase(string $string): bool
     /**
      * Returns true if the given string is upper case, false otherwise.
      *
-     * @param [type] $string
+     * @param string $string
      *
      * @return bool
      */
-    public static function isUpperCase($string): bool
+    public static function isUpperCase(string $string): bool
     {
         return $string === mb_strtoupper($string);
     }
@@ -106,11 +106,11 @@ public static function isUpperCase($string): bool
     /**
      * Returns true if the given string is a palindrome, false otherwise.
      *
-     * @param [type] $string
+     * @param string $string
      *
      * @return bool
      */
-    public static function palindrome($string): bool
+    public static function palindrome(string $string): bool
     {
         return strrev($string) === $string;
     }
@@ -147,12 +147,12 @@ public static function countVowels(string $string): int
      * Decapitalizes the first letter of the sring and then adds it with rest of the string. Omit the upperRest parameter to keep the
      * rest of the string intact, or set it to true to convert to uppercase.
      *
-     * @param [type] $string
+     * @param string $string
      * @param bool $upperRest
      *
      * @return string
      */
-    public static function decapitalize($string, $upperRest = false): string
+    public static function decapitalize(string $string, bool $upperRest = false): string
     {
         return mb_strtolower(mb_substr($string, 0, 1)) . ($upperRest ? mb_strtoupper(mb_substr($string, 1)) : mb_substr($string, 1));
     }
@@ -161,7 +161,7 @@ public static function decapitalize($string, $upperRest = false): string
      *  Substring a string to specific lenght, but removing whole words
      *
      * @param string $string
-     * @param string $to
+     * @param int $to
      *
      * @return string
      */
@@ -189,6 +189,6 @@ public static function letterPlusNumber(string $letter, int $number): string
             ++$letter;
         }
 
-        return $letter;
+        return (string) $letter;
     }
 }